1. Field Of The Invention
The invention relates to a method and apparatus capable of rotating a spherical object on any axis through its center with the axis varying as a function of time such as to cause the object's surface to move past a selected point above the surface in a path.
2. Prior Art
In the manufacture of spherical components, such as, bearing balls used in the automotive, aerospace and aircraft industries, it is often necessary to inspect the condition and appearance of the ball for defects which would be detrimental to their performance. Electromagnetic, ultrasonic, optical, x-ray and other instrumental techniques are used for such inspection. It is generally desirable to inspect the entire surface using small sensors or probes with sensing areas very much smaller than the total ball surface. Inspection of the complete surface requires manipulating the sensor around a stationary ball or manipulating the ball surface past a stationary sensor or probe or a combination of such motions. In any case, controlled motion is required in order to realize complete inspection coverage of the entire surface.
Similarly, in other manufacturing operations, controlled motion of a spherical surface for treatment, marking, or applications of other materials in pre-determined patterns is required. For such applications also, either an applicator or spherical object or both are required to be manipulated in order to complete the process.
In one prior method (U.S. Pat. No. 3,398,592) controlled movement of balls is accomplished by suspending the ball at four points between a combination of conical and cylindrical elements with one of the elements being a driven rotating member. One or two the others are rollers of eccentric conical sections or irregular cylindrical sections axially fixed connected by a cylindrical coaxial portion of a shaft which is journaled in bearings (see FIGS. 11 to 13 and 16A to 16h) whose dimension or shape are such as to cause the driven ball to rotate on a moving axis in order to provide a complete scan in front of a fixed reference point. A major disadvantage of this method is that sets of different rotating elements of complex geometry are required for each ball size to be tested thereby requiring maintaining a large inventory of elements and changing of the elements for each ball to be manipulated or maintaining an inventory of complete machanisms for each ball size. Another disadvantage is that only one scan pattern may be realized with a given set of elements thereby precluding the capability to provide various scan patterns with only one set of elements.
The prior art section of U.S. Pat. No. 3,398,592 states:
"In known methods for rotating balls, the ball is rolled between two conical driving elements which, in addition to a basic rotary movement about a common axis, also move relative to each other." PA0 "When the known method is applied to ball of relatively large diameter requiring a great number of meridians, it is impossible even in precisely built devices adequately to inspect areas at the poles of the meridian pattern."
In another prior technique (U.S. Pat. No. 4,551,677), specifically for the eddy-current testing of balls, the method uses an inverted cone-shaped cup placed over the top of a ball which in turn is placed on a flat, high-friction surface. Friction between the ball and the cone-shaped cup is reduced by creating a surface of pressurized air between the ball and the cup via an air injection nozzle on the cup. With the ball so clamped between the cup and the flat surface, the cup is made to move over the surface in a predetermined manner such as to cause each point of the ball surface to move and pass by an eddy-current probe mounted on the cone-shaped cup. The preferred embodiment is to move the cup in an X-Y raster scan pattern over the front surface with movements between each X and Y direction traversal fixed in order to affect a global scan pattern with equally spaced meridian lines. Two disadvantages of this method are (a) requirement for complex mechanics for high-speed inspection of balls and (b) limited ball-size range for a given set of cup dimensions.
In still another method used specifically for inspection of bearing balls (U.S. Pat. No. 4,430,614), the ball is held by vacuum on the end of a hollow cylinder which rotates in one direction. A probe is placed above the ball at a point perpendicular to the axis of the cylinder such that as the cylinder (and ball) rotates, the trajectory of the probe is a great circle. The probe is then made to move in an arc in the plane of the holding cylinder such that the probe trajectory is a series of circles of diminishing diameter thereby effecting a helical scan pattern over a portion of the ball surface. Using a second hollow cylinder under vacuum, the ball is removed from the first cylinder, the second cylinder rotates 90.degree., the ball then replaced on the first cylinder and the process repeated to affect the helical scan over the previously uncovered portion of the ball. Two disadvantages of this method are (a) double manipulation of each ball requiring longer time to complete the scan and more complex apparatus and (b) the velocity of the inspection point on the surface with respect to the probe decreases as the probe travels an arc and goes to near zero at the pole of the scan.
In a non-confidential telephone communication by one of the inventors with an employee of NASA, the NASA employee stated that in research done prior to publication of NASA Tech Brief, Viol. 8, No. 3, MFS-25833 and filing for U.S. Pat. No. 4,551,677, attempts were made to scan the surfaces of bearing balls utilizing two fixed-speed motors to rotate the ball on two perpendicular axes but it was found that this method would not yield continuous rotation of the axis of rotation. Further attempts were made using three independently driven fixed-speed motors but it proved too difficult to precisely control the motor speeds to effect an accurate scan.
For ultrasonic inspection of balls, other methods are known (U.S. Pat. Nos. 4,281,548 and 4,387,596) in which a ball is held in a spherical cavity with the entire assembly submerged in a liquid coupling medium suitable for ultrasonic inspection. One or two liquid jets introduced into the side walls of the spherical cavity are directed such as to cause the ball to rotate precessionally or multi-axially in the presence of ultrasonic transducers directed at the ball. The requirements to submerge the entire apparatus in a liquid medium limits the application of types of objects to be inspected, slows down the inspection cycle and further presents a possible contamination problem when the item to be inspected cannot be cleaned before and after immersion in the liquid medium.
Other methods have been attempted by applicants in which two or more rotating elements were used to manipulate a ball held in a stationary position with respect to the axis of rotation of the rotating members. In these cases, the rotating elements were driven at fixed differential speeds. It was ultimately shown that under such conditions, the entire surface of the sphere could not be moved past a fixed point in a controlled manner.